High-Temperature Interband Cascade Lasers

ABSTRACT

An interband cascade gain medium is provided. The gain medium can include at least one thick separate confinement layer comprising Ga(InAlAs)Sb between the active gain region and the cladding and can further include an electron injector region having a reduced thickness, a hole injector region comprising two hole quantum wells having a total thickness greater than about 100 Å, an active gain quantum well region separated from the adjacent hole injector region by an electron barrier having a thickness sufficient to lower a square of a wavefunction overlap between a zone-center active electron quantum well and injector hole states, and a thick AlSb barrier separating the electron and hole injectors of at least one stage of the active region.

CROSS-REFERENCE

This application is a continuation of and claims the benefit of prioritybased on U.S. patent application Ser. No. 12/402,627 filed on Mar. 12,2009, which in turn is a non-provisional of and claims the benefit ofpriority based on U.S. Provisional Patent Application No. 61/106,693filed on Oct. 20, 2008, both of which are hereby incorporated byreference into the present application.

TECHNICAL FIELD

The present invention relates to an interband cascade gain medium andinterband cascade lasers incorporating such a medium for improved laseror optical amplifier performance in the mid-infrared range attemperatures accessible with thermoelectric cooling or above.

BACKGROUND

There has been an increasing interest in the development of lasersources that emit in the mid-infrared (“mid-IR”) spectral region, i.e.,at wavelengths between about 2.5 and 8 μm. Such lasers have significantuses for both military and non-military applications. In the militaryrealm, mid-IR lasers can be extremely useful as a countermeasure to jamheat-seeking missiles and prevent them from reaching their targets. Inboth the military and non-military realm, such mid-IR lasers have founduse, for example, in chemical sensing, and so may be very useful inenvironmental, medical, and national security applications.

On the short-wavelength side of this spectral region, type-Iquantum-well antimonide lasers are achieving excellent performance andgreater maturity. See, e.g., L. Shterengas et al., “Continuous waveoperation of diode lasers at 3.36 μm at 12° C.,” Appl. Phys. Lett. 93,011103 (2008). On the long-wavelength side of the mid-IR, intersubbandquantum cascade lasers (QCLs) have become the dominant source of laseremissions. See, e.g., S. Slivken, et al., Compound Semiconductors(October 2008), at p. 21.

For the mid-infrared spectral region, the interband cascade laser (ICL)is being developed as a promising semiconductor coherent source.

The first ICLs were developed by Rui Yang in 1994. See U.S. Pat. No.5,588,015 to Yang. The ICL may be viewed as a hybrid structure, whichresembles a conventional diode laser in that photons are generated viathe radiative recombination of an electron and a hole. However, it alsoresembles a quantum cascade laser in that multiple stages are stacked asa staircase, such that a single injected electron can produce anadditional photon at each step of the staircase. See Slivken et al.,supra; see also U.S. Pat. No. 5,457,709 to Capasso et al. This photoncascade is accomplished by applying a sufficient voltage to lower eachsuccessive stage of the cascade by at least one photon energy, andallowing the electron to flow via an injector region into the next stageafter it emits a photon. An interband active transition requires thatelectrons occupying states in the valence band (following the photonemission) be reinjected into the conduction band at a boundary withsemi-metallic or near-semi-metallic overlap between the conduction andvalence bands. Outside of the active quantum well region and holeinjector, transport in the ICL typically takes place entirely via themovement of electrons, although this is not required. Therefore, twooptical cladding regions are generally used at the outsides of the gainmedium to confine the lasing mode along the injection axis, and n-typecontacts are provided outside the cladding regions to provide forelectrical bias and current injection.

ICLs also employ interband active transitions just as conventionalsemiconductor lasers do. Although type-I ICLs are also possible (seeU.S. Pat. No. 5,799,026 to Meyer et al., two inventors of which are theinventors of the present invention, and which is incorporated byreference into the present disclosure), most ICLs employ activetransitions that are of type-II nature, i.e., the electron and holewavefunctions peak in adjacent electron (typically InAs) and hole(typically Ga(In)Sb) quantum wells, respectively. In order to increasethe wavefunction overlap, two InAs electron wells often are placed onboth sides of the Ga(In)Sb hole well, and create a so-called “W”structure. In addition, barriers (typically Al(GaInAs)Sb) having largeconduction- and valence-band offsets can surround the “W” structure inorder to provide good confinement of both carrier types. See U.S. Pat.No. 5,793,787 to Meyer et al., which shares an inventor in common withthe present invention and which is incorporated by reference herein.Further improvements to the basic ICL structure, such as including morethan one hole well to form a hole injector, were subsequently made bythe present NRL inventors and Dr. Yang. See U.S. Pat. No. 5,799,026 toMeyer et al., supra.

Despite these improvements, the performance of the first ICLs fell farshort of the theoretical expectations. In particular, the thresholdcurrent densities at elevated temperatures were quite high (5-10 kA/cm²at room temperature in pulsed mode), which precluded continuous-wave(cw) operation of those devices at temperatures higher than ≈150 K.Later work by researchers at Army Research Laboratory, Maxion, and JetPropulsion Laboratory further improved the operation of ICLs. See e.g.,R. Q. Yang et al., “High-power interband cascade lasers with quantumefficiency>450%,” Electron. Lett. 35, 1254 (1999); R. Q. Yang, et al.,“Mid-Infrared Type-II Interband Cascade Lasers,” IEEE J. Quant.Electron. 38, 559 (2002); and K. Mansour et al., “Mid-infrared interbandcascade lasers at thermoelectric cooler temperatures,” Electron. Lett.42, 1034 (2006). However, performance in the mid-IR at room temperaturestill remained unsatisfactory.

SUMMARY

This summary is intended to introduce, in simplified form, a selectionof concepts that are further described in the Detailed Description. Thissummary is not intended to identify key or essential features of theclaimed subject matter, nor is it intended to be used as an aid indetermining the scope of the claimed subject matter. Instead, it ismerely presented as a brief overview of the subject matter described andclaimed herein.

The present invention comprises an interband cascade gain medium and aninterband cascade laser or amplifier having the same. An interbandcascade gain medium in accordance with the present invention can includeany one or more of the following features: (1) the active quantum wellregion includes a thick and In-rich GaInSb hole well; (2) the holeinjector includes two or more Ga(InAlAs)Sb hole wells having thicknessesin a specified range; (3) the electron and hole injectors are separatedby a thick Al(GaInAs)Sb barrier to suppress interband absorption; (4)the thickness of the first electron barrier of the hole injector regionseparating the hole injector region from an adjacent active quantum wellregion is sufficient to lower a square of a wavefunction overlap betweena zone-center active electron quantum well and injector hole states tonot more than 5%; (5) the thickness of the first InAs electron well inthe electron injector, as well as the total thickness of the electroninjector, is reduced; (6) the number of cascaded stages is reduced from10 or more to the range between 2 and 7; (7) transition regions areinserted at the interfaces between the various regions of the gainmedium so as to smooth out abrupt shifts of the conduction-band minimum;(8) thick separate confinement layers comprising Ga(AlInAs)Sb aredisposed between the active gain region and the cladding to confine theoptical mode and increase its overlap with the active stages; and (9)the doping profile of the superlattice (SL) cladding layers is optimizedto minimize the overlap of the optical mode with the most heavily-dopedportions of the cladding layers.

In accordance with the present invention, an interband cascade gainmedium can employ one or more of the described features, and a laserincorporating such a gain medium can emit in the mid-IR range from about2.5 to 8 μm at high temperatures with improved continuous waveperformance and greater efficiencies.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram depicting a layer structure of an exemplaryinterband cascade gain medium in accordance with the present invention.

FIG. 2A depicts the conduction and valence band profiles of an exemplaryactive gain region of an interband cascade gain medium in accordancewith the present invention.

FIG. 2B depicts selected electron and hole energy levels andwavefunctions corresponding to the conduction and valence band profilesshown in FIG. 2A.

FIG. 3 depicts continuous wave light-current-voltage characteristics ofan exemplary embodiment of a laser comprising an interband cascade gainmedium in accordance with the present invention.

FIG. 4 is a plot of Auger coefficients vs. wavelength for various type-Iand type-II semiconductor materials, including interband cascade gainmedia in accordance with the present invention.

DETAILED DESCRIPTION

The invention summarized above can be embodied in various forms. Thefollowing description shows, by way of illustration, combinations andconfigurations in which the aspects can be practiced. It is understoodthat the described aspects and/or embodiments of the invention aremerely examples. It is also understood that one skilled in the art mayutilize other aspects and/or embodiments or make structural andfunctional modifications without departing from the scope of the presentdisclosure.

For example, although the gain medium is described herein as comprisingsemiconductor layers of specified thicknesses arranged in a specifiedconfiguration, one skilled in the art would appreciate that other layerthicknesses and configurations may also be used. In addition, althoughthe gain medium according to the present invention is described hereinas comprising InAs, GaInSb, GaSb, and AlSb semiconductor materials, oneskilled in the art will appreciate that other semiconductor materialsmay be substituted. In some embodiments, a small amount of In can beintroduced into some of the AlSb layers for strain compensation, andsuch a case is denoted as having the structure “Al(In)Sb.” For a lowerbarrier one may also introduce Ga to form Al(Ga)Sb. A small amount of Asmay also be added, to form Al(GaAs)Sb, in order to adjust the latticeconstant for lattice matching or strain compensation. A more generalalloy with barrier properties playing a similar role to AlSb isAl(GaInAs)Sb. Similarly, a small amount of Al and/or As can beintroduced into the Ga(In)Sb; such cases are denoted as having thestructure “Ga(Al)Sb,” “Ga(As)Sb,” and “Ga(AlAs)Sb,” respectively. A moregeneral alloy that can serve as a hole quantum well is Ga(AlInAs)Sb.Whenever an alloy composition is specified, such as Ga_(1-x)In_(x)Sb, itis understood that the composition x may be zero (making the materialGaSb in this example).

The present invention comprises an interband cascade gain medium and aninterband cascade laser or amplifier using the same which can emit inthe mid-IR range from about 2.5 to about 8 μm at high temperatures withimproved continuous wave performance and greater efficiencies. Sincemany of the previous semiconductor lasers operating in this wavelengthregion have required cryogenic cooling, which is impractical for mostapplications, in the following description, the term “high temperature”will refer to temperatures of about 250 K and above, which can beaccessed with a practical thermoelectric cooler or without any activecooling.

In accordance with the present invention, an interband cascade gainmedium comprises a series of cascaded stages, each stage including anactive gain region having an active quantum well region, a holeinjector, and an electron injector comprising an InAs/Al(In)Sb SL. Ateach stage the active gain region can include at least one of thefollowing features, which are described in more detail below: (1) theactive quantum well region includes a thick and In-rich GaInSb holewell; (2) the hole injector includes two or more Ga(InAlAs)Sb hole wellshaving thicknesses in a specified range; (3) the electron and holeinjectors are separated by a thick Al(GaInAs)Sb barrier to suppressinterband absorption; (4) the thickness of the first electron barrier ofthe hole injector region separating the hole injector region from anadjacent active quantum well region is sufficient to lower a square of awavefunction overlap between a zone-center active electron quantum welland injector hole states to not more than 5%; (5) the thickness of thefirst InAs electron well in the electron injector, as well as the totalthickness of the electron injector, is reduced; and (6) the number ofcascaded stages is reduced from 10 or more to the range between 2 and 7;(7) transition regions are inserted at the interfaces between thevarious regions of the gain medium so as to smooth out abrupt shifts ofthe conduction-band minimum; (8) a thick separate confinement layercomprising Ga(AlInAs)Sb is disposed at each end of the active gainregion between the active gain region and an outer cladding layer toconfine the optical mode and increase its overlap with the activestages; and (9) the doping profile of the cladding layers is optimizedto minimize the overlap of the optical mode with the most heavily-dopedportions of the cladding layers.

As described herein, an interband cascade gain medium in accordance withthe present invention can have any one or more of features (1)-(9),either alone or in combination with any other of features (1)-(9). Inaccordance with the invention, each stage of the gain medium can havethe same or different features or the same or different combination offeatures, and all such configurations are within the scope of thepresent disclosure.

The present invention also can include an interband cascade laser and anexternal cavity laser employing a gain medium having one or more of thefeatures described above. See K. Mansour et al., supra; R. Maulini etal., “Widely tunable high-power external cavity quantum cascade laseroperating in continuous-wave at room temperature Electronics Letters 45,107 (2009)]. In accordance with the present invention, such an interbandcascade or external cavity laser can emit in the mid-IR range from about2.5 to 8 μm at temperatures at or above those accessible withthermoelectric cooling, with improved continuous wave performance andgreater efficiency. In addition, the present invention also can includean interband cascade amplifier employing a gain medium having one ormore of the features described above, and in accordance with the presentinvention, such an amplifier can amplify light in the mid-IR range fromabout 2.5 to 8 μm at temperatures at or above those accessible withthermoelectric cooling, with improved continuous wave performance andgreater efficiency. See M. J. Connelly, Semiconductor Optical Amplifiers(Boston, Springer-Verlag, 2002).

A block diagram of an exemplary configuration of an interband cascadegain medium in accordance with the present invention is depicted in FIG.1, where like-patterned areas are used to denote like or very similarmaterials.

As shown in FIG. 1 and as described herein, an interband cascade gainmedium in accordance with the present invention can comprise a series ofstacked layers of semiconductor material which can form a series ofquantum barriers and wells that control the movement of electrons andholes in the medium. As shown in FIG. 1, an interband cascade gainmedium in accordance with the present invention can include an activegain region comprising a series of cascaded stages, each stage 101comprising an active quantum well (QW) region 101 a, a hole injector 101b, and an electron injector 101 c. The components of the active gainregion act in combination to produce electron and hole energy levels andwavefunctions which, when combined under appropriate bias and electricalinjection, cause the emission of light from the medium. The individualstages 101 of the active gain region are repeated a number of times tocomprise the gain medium. As noted above, each iteration of theindividual stages 101 can include one or more of the features describedherein, with each individual stage including the same or differentfeatures as the other stages in the gain medium.

The gain medium further can include separate confinement layers (SCLs)106 a and 106 b which can be located at each end of the active gainregion, cladding layers 108 a and 108 b, and a substrate 111, with afirst transition region 107 a disposed between SCL 106 a and cladding108 a, a second transition region 107 b between SCL 106 b and cladding108 b, a third transition region 107 c between cladding layer 108 b andsubstrate 111, a fourth transition region 107 d between active gainregion 101 and SCL 106 a, and a fifth transition region 107 e betweenactive region 101 and SCL 106 b, to smooth out the abrupt voltage shiftin the conduction band profiles of the two adjoining regions. In someembodiments (not shown), an additional contact layer can be alsodisposed between substrate 111 and cladding layer 108 b. In addition, ann⁺-InAs or other suitable top contact layer 110 can be disposed at thetop of the epitaxial structure, with a second transition region 109disposed between the top contact layer 110 and the top cladding layer108 to again smooth out the otherwise-abrupt shift of the conductionband profile.

In accordance with aspects of operation of semiconductor interbandlasers known in the art, the structure of a gain medium in accordancewith the present invention produces conduction and valence band energiesand corresponding electron wavefunctions that govern the movement andrecombination of electrons and holes in the semiconductor materials andso govern the creation and emission of photons by the laser.

FIGS. 2A and 2B are block diagrams showing plots of the conduction bandprofiles (E_(c)) and valence band profiles (E_(v)) in an exemplary stageof the active gain region, along with the beginning of the next stage,of a gain medium incorporating one or more of the features describedbelow in accordance with the present invention.

As seen in FIGS. 2A and 2B, each stage of the active gain regionconsists of the electron and hole active QWs 201, a hole injector 202,and an electron injector 203, with a new stage, again beginning withelectron and hole active QWs 201, cascading from electron injector 203.

In accordance with principles of semiconductor interband cascade lasersknown in the art, electrons injected from an electron pump sourcepopulate conduction states in the active QWs 201. In the configurationshown in FIGS. 2A and 2B, the electrons are injected from left to right,although of course other directionalities of the electron source arepossible. Photons are emitted via interband optical transitions from theconduction states to the valence states in the active QWs. Once in theactive QW valence band, electrons tunnel to the first (and then thesecond, if it is present) Ga(InAlAs)Sb QW(s) 202 b of the hole injector202. Because of the type-II energy overlap between InAs andGa(InAlAs)Sb, the key feature that allows cascading, electrons in theGa(InAlAs)Sb valence band elastically scatter or tunnel toconduction-band states in the relatively-thick first InAs QW 203 a ofthe electron injector 203. Once back in the conduction band, theelectrons propagate through the InAs/Al(In)Sb SL havingprogressively-decreasing QW thickness that comprises the electroninjector 203 to regain energy before tunneling through a final Al(In)Sbbarrier 203 b into the next active QWs 201 for recycling and theemission of additional photons. The cascading of multiple stagesprovides multiple photons (more power) out for every electron in at theexpense of a higher bias voltage required to charge all of the activegain regions simultaneously. This trade is generally advantageous, sincelower current for a given output power means that parasitic ohmic andnon-ohmic voltage drops become relatively less important.

In accordance with the present invention, the operation of these aspectsof an interband cascade gain medium can be improved by the incorporationof one or more of Features (1)-(9) described below.

Feature (1)—Ga_(1-x)In_(x)Sb Active Hole Quantum Well

As noted above, electrons and holes recombine in the active QWs 201 toproduce photons. However, as is known in the art, in some cases theenergy from the electron-hole pair does not produce a photon, butinstead is transferred to another electron or hole, a process known asAuger recombination. Thus, one goal in laser design is to reduce theAuger recombination in order to reduce the non-radiative decay andthereby reduce the lasing threshold.

Some theories have predicted that the performance of type-II antimonidelasers such as those having the “W” or ICL configuration should dependstrongly on the exact layering details, due to the potential forresonances between the energy gap and valence intersubband transitions.Such resonant processes could potentially degrade the laser performancedue to increased free carrier absorption and Auger recombination.However, NRL investigations have not revealed any clear evidence forresonant processes, since the experiments show no obvious correlationbetween ICL performance and the details of the layering sequence in theactive QW region. Nevertheless, the data show a general non-resonanttrend towards lower ICL current-density thresholds and higherphoton-production efficiencies as the In composition and thickness ofthe active hole QW 201 a are increased.

Thus, in accordance with feature (1) of the present invention, activeQWs 201 can include a Ga_(1-x)In_(x)Sb hole quantum well 201 asurrounded by InAs electron quantum wells 201 b and 201 c. In accordancewith the present invention, the Ga_(1-x)In_(x)Sb hole QW 201 a has acomposition x that is as large as possible while maintaining high growthquality (e.g., as characterized by morphology and x-ray linewidths) andcan have a thickness of about 25 Å to about 50 Å. In an exemplaryembodiment, active QWs 201 employ a Ga_(1-x)In_(x)Sb hole QW ofcomposition x=0.35 and thickness 30 Å. In addition, the thicknesses ofthe two adjacent InAs electron QWs 201 b and 201 c can be adjusted so asto produce the desired emission wavelength.

Analysis of the ICL threshold data has shown that a gain medium havingactive QWs 201 with this feature in accordance with the presentinvention have a smaller Auger coefficient and exhibit lower ICLcurrent-density thresholds at higher temperatures near ambient.

Feature (2)—Hole Injector Having Two Ga(InAlAs)Sb Hole Quantum Wells

As noted above, each active gain stage 101 of an interband cascade gainmedium can include a hole injector 202 to prevent the tunneling ofelectrons in the active electron QWs 201 b and 201 c directly into theelectron injector 203. As shown in FIGS. 2A and 2B, hole injector 202can comprise a series of alternating hole QWs 202 b and barriers 202 a,202 c, and 202 d, where each barrier 202 a, 202 c, and 202 d can beeither AlSb or AlGaInAsSb and each hole QW 202 b can be either GaSb orGa(InAlAs)Sb. In accordance with feature (2) of the present invention,hole injector 202 can comprise two Ga(InAlAs)Sb hole QWs 202 b as shownin FIGS. 2A and 2B, each of which has a thickness greater than that usedin prior structures, with a total thickness for both hole QWs exceedingabout 100 Å. Thus, in an exemplary embodiment, hole injector 202 cancomprise AlSb/GaSb/AlSb/GaSb/AlSb layers having the conduction andvalence band energy levels and corresponding wavefunctions shown inFIGS. 2A and 2B. One may also reach the total thickness of 100 Å byemploying more than two hole QWs.

In accordance with the present invention, this feature inhibits electrontunneling (either coherent or incoherent) between the active electronQWs 201 b and 201 c and the electron injector 203. If this parasitictunneling is negligible, almost all electrons are transported via thevalence-band states in the hole injector 202, ideally after making aradiative transition. The wavefunction overlap between electron statesin the active electron QWs 201 b and 201 c seen in FIG. 2B, and theelectron states in the initial portion of the electron injector 203 aunder these conditions is negligible, and the barrier is also thickenough to effectively eliminate trap-assisted tunneling.

An exemplary structure of a hole injector 202 having two Ga(InAlAs)Sbhole QWs 202 b in accordance with the present invention comprises 10 ÅAlSb/50 Å GaSb/10 Å AlSb/70 Å GaSb/20 Å AlSb. The first AlSb barrier 202a shown in FIGS. 2A and 2B separates the InAs/GaInSb/InAs “W” active QWs201 from hole injector 202. In accordance with the present invention,barrier 202 a should be thick enough to lower the square of thewavefunction overlap between the zone-center active electron andinjector hole states to <5%, so as to reduce the rate of any interbandtransitions not involving the active QW region.

Feature (3)—AlSb Barrier Between Hole Injector and Electron Injector

The last AlSb barrier in the structure of hole injector 202 describedabove is barrier 202 d shown in FIGS. 2A and 2B and is between holeinjector 202 and InAs/AlSb electron injector 203. This is the point atwhich electrons in the valence band (following the photon emission) maketheir transition back to the conduction band. While it had been thoughtthat this barrier should be no thicker than ≈15 Å to allow an adequateinterband scattering rate, in accordance with feature (3) of the presentinvention, the thickness of AlSb barrier 202 d can be increased to 20-25Å. Recent NRL data confirm that this reduces the interband absorption,which was found to be a larger effect than previously believed, whilestill allowing adequate transport from the valence band to theconduction band.

To be optimal, this barrier thickness should lead to an interbandoptical matrix element no greater than 0.2 eV Å at the zone center,while retaining a wavefunction overlap of at least 0.3% to maintainadequate interband scattering.

Feature (4)—Thickness of Electron Barrier Lowering of WavefunctionOverlap

As described above with respect to FIG. 1 and as seen in FIGS. 2A and2B, the active gain region comprises a series of alternating barriersand quantum wells. As seen in FIGS. 2A and 2B, hole injector 202 isseparated from an adjacent active quantum well region 201 by an electronbarrier 202 a.

In accordance with this feature (4) of the present invention, this firstelectron barrier 202 a of hole injector region 202 can have a thicknesssufficient to lower a square of a wavefunction overlap between azone-center active electron quantum well and injector hole states to notmore than 5%.

Feature (5)—Reduction in Thickness of the InAs/Al(in)Sb ElectronInjector

As described above with respect to FIG. 1 and as seen in FIGS. 2A and2B, electron injector 203 comprises a series of alternating barriers andquantum wells. The thickness of the wells is graded so as to compensatethe applied field with quantum confinement, resulting in the formationof a miniband that allows rapid electron transport through the electroninjector. In accordance with this feature (5) of the present invention,the total thickness of the InAs/AlSb electron injector 203 of the gainmedium can be reduced considerably, from 400-500 Å in earlier designs to200-250 Å.

As noted above, the thicknesses of the wells in electron injector 203are graded, and so in some embodiments the reduction in total thicknesscan largely be achieved by substantially reducing the thickness of thefirst QW 203 a. Thus, in an exemplary embodiment of a gain materialhaving features in accordance with the present invention, the thicknessof the first QW 203 a is reduced, from 70-100 Å in earlier designs to40-50 Å.

In addition, as described above, electron injector 203 comprises anumber of alternating quantum wells and barriers. In accordance with thepresent invention, an electron injector can comprise from 4 to 9 InAsquantum wells separated by Al(In)Sb barriers each having a thickness of≈12 Å, where the optimum number of wells is related to the desiredemission wavelength. For shorter wavelengths, more wells are used sincethe voltage drop per stage is roughly proportional to the photon energy.On the other hand, fewer wells are employed for longer-wavelength ICLs.However, the wells near the next stage are much thinner forshorter-wavelength, structures, so that the total thickness of electroninjector 203 does not vary as much.

Thus, an exemplary embodiment of an electron injector having features inaccordance with the present invention for an emission wavelength of 4.3μm at 300 K can have alternating barriers and wells comprising thefollowing structure: 42 Å InAs/12 Å AlSb/33 Å InAs/12 Å AlSb/28 ÅInAs/12 Å AlSb/25 Å InAs/12 Å AlSb/23 Å InAs/12 Å AlSb/23 Å InAs/25 ÅAlSb.

In addition, in accordance with this feature of the present invention,the final AlSb barrier 203 b between electron injector 203 and the nextcascaded active QW region 201 can be much thicker than the others in theelectron injector to prevent strong hybridization of the lasing electronsubband in the active QW region 201 with the miniband of the injector203.

The advantages of a thinner electron injector in accordance with thisfeature of the present invention are twofold. First, the thinnerelectron injector results in a reduction in the density of states of theelectrons in the electron injector, which causes fewer holes to beinjected into the hole injector and, subsequently, into the active QWregion and thus reduces the internal loss due to free-hole absorption.Second, the average refractive index of the active gain region and theinjectors increases because low-index InAs/Al(In)Sb layers now form asmaller fraction of the total active gain region and high-indexGa(InAlAs)Sb layers form a larger fraction, which results in betteroptical confinement.

When these advantages are taken together, Feature (5) results in a lowerinternal loss due to free hole absorption as well as a higher opticalconfinement factor.

Feature (6)—Small Number of Cascaded Stages

As described above, an interband cascade gain medium includes an activegain region comprising a number of cascaded stages, each cascaded stagehaving active QWs 201, a hole injector 202, and an electron injector203.

Earlier ICLs fabricated at University of Houston, Army ResearchLaboratory, Maxion, and JPL always employed at least 12 and as many as35 stages. Such a large number of stages was required in part becausethose lasers had higher internal losses than the recent NRL ICLs thatemploy the invention.

However, because of the reduction in internal optical losses associatedwith use of one or more of the features described herein, the number ofsuch cascaded stages in the gain medium can be reduced. Thus, dependingon the modal loss and the gain required, in accordance with feature (6)of the present invention, an interband cascade gain medium in accordancewith the present invention can comprise between 2 and 7 cascaded stages,significantly fewer than the 12-35 stages used in prior ICLs. Theoptimal number of stages is chosen so that each stage provides from 3 to8 cm⁻¹ of modal gain (material gain multiplied by the opticalconfinement factor) in order to produce enough gain to reach thethreshold for lasing at room temperature, before thermal runawayprevents the occurrence of lasing.

Earlier NRL structures usually employed either 5 or 10 stages, althoughthe older 5-stage devices performed poorly at higher temperaturesbecause their losses were too high for the gain to be adequate. However,NRL testing has confirmed that when the internal loss is reduced, forexample, through employment of a graded doping scheme such as thatdescribed below, the high-temperature performance in continuous-wavemode can be substantially improved when 5 stages are employed ratherthan 10. One recent NRL structure achieved favorable performance withonly 3 stages.

Because a significant fraction of the loss comes from the active layers,reducing the number of stages can further reduce the loss and thereforeimprove the slope and wallplug efficiencies. In addition, reducing thenumber of stages in the gain material can also lower lasing thresholds,further contributing to the efficiency of a laser using such a material.

Feature (7)—Graded Transition Regions to Smooth Out Abrupt Shifts of theConduction Band Minimum

According to feature (7) of the present invention, specially-designedgraded transition regions can be employed to smooth out banddiscontinuities between the various regions of the waveguide that havemisaligned conduction band minima.

In the presence of a significant conduction band discontinuity, a largepotential barrier forms at the interface between two regions and impedesthe carrier transport. This can result in an extra voltage drop thatrequires additional heat dissipation and lowers the device's wallplugefficiency. However, if each discontinuity is smoothed out via theincorporation of a graded transition region with a conduction-bandposition that gradually varies between those of the two regions that areto be joined, the height and spatial extent of the potential barriersare greatly reduced, as is the parasitic voltage drop.

In an exemplary embodiment of an interband cascade gain medium havingthis feature, a series of graded InAs/AlSb and InAs/AlSb/GaSb transitionsuperlattices, all of which are strain-compensated to the GaSb latticeconstant, can be used to gradually grade the height of the conductionband minimum.

An exemplary embodiment can employ transitions between the active gainregion and the SCLs. In such an embodiment, the transition layers cancomprise 8 periods of 20.1 Å InAs/19 Å AlSb, 10 periods of 15.9 ÅInAs/15 Å AlSb, and 12 periods of 7 Å InAs/6.4 Å AlSb/15 Å GaSb. Thesame graded transition layers can be used between the GaSb buffer layeror substrate and the bottom optical cladding layer, and between eitherof the optical cladding layers and the adjacent SCL.

In addition, in some embodiments, the following graded transition layerscan be used between the top cladding region and the n⁺-InAs cap layer,starting from the cladding: 7 periods of 29 Å InAs/14 Å AlSb, 6 periodsof 42 Å InAs/12 Å AlSb, and 4 periods of 65 Å InAs/10 Å AlSb. Thesetransition layers between the cladding and the cap can be used eitherwith or without the transition layers between the active gain region andthe SCLs.

In some embodiments, these graded transition regions are doped to alevel of 0.5−5×10¹⁷ cm⁻³, with the higher doping level being employed inregions of low overlap with the lasing optical mode. A lower dopinglevel may be employed in regions of high overlap with the lasing mode,although a higher level can be used there as well to further minimizevoltage barriers and to reduce the susceptibility to damage at highinjection currents.

Feature (8)—Thick High-Index Ga(AlInAs)Sb Separate Confinement Layer

Since the average refractive index of the active stages is not veryhigh, they cannot by themselves confine a guided optical mode when asmall number of active stages is employed.

Feature (8) can be employed in an interband cascade gain medium inaccordance with the present invention to remedy this effect. Inaccordance with this feature of the present invention, a high-indexGa(AlInAs)Sb separate confinement layer (SCL) can be used to confine theoptical mode and increase its overlap with the active stages. By makingthe SCLs thick, this feature can also substantially lower the modalloss. Since typically the Ga(AlInAs)Sb of the separate confinement layer(SCL) has much lower material loss than the superlattices of the activeand cladding layers, the net modal loss is lower if a large fraction ofthe mode resides in the SCL.

In an exemplary embodiment of the use of this feature in an interbandcascade gain medium according to the present invention, a 200-nm-thickGa(AlInAs)Sb SCL is positioned both above and below the active gainregion of the gain medium. In some embodiments, the SCLs can be dopedn-type to a low level of <2×10¹⁷ cm⁻³, although doping should be highenough to compensate for the usual p-type background doping of the SCLmaterial. In some embodiments, GaSb can be used for the SCLs, althoughadding some AlAs to form Al_(x)Ga_(1-x)As_(y)Sb_(1-y) (lattice-matchedto GaSb) can also be used to lower the refractive index of the SCL andthereby increase the modal overlap with the active stages. In addition,AlGaInAsSb or AlGaSb layers with a slight lattice mismatch with respectto the GaSb substrate can also be used.

In some embodiments of the interband cascade gain medium having thisfeature, the SCL layers can be quite thick, e.g., 0.4-1 μm for a totalthickness of 0.8-2 μm for both SCLs, in order to concentrate asubstantial fraction of the optical mode in a region with very lowoptical losses. In any case, however, the SCLs must not be so thick thatlasing occurs in a higher-order vertical mode rather than thefundamental mode, since such a mode would have wider beam divergence andbe more susceptible to multi-mode lasing.

Feature (9)—Graded Doping Profile in the Cladding Layers

In accordance with this feature (9) that can be used in an interbandcascade gain medium in accordance with the present invention, a gradeddoping profile can be used in one or more of the semiconductor materiallayers comprising the gain medium.

For example, in an exemplary embodiment of a gain medium having thisfeature, in the ≈1-1.5 μm portion of the bottom and top cladding layersthat are adjacent to the active gain region (or SCL if such a separatelayer according to Feature (8) is used), the doping level can belowered, for example, to 0.5−2×10¹⁷ cm⁻³, whereas the remaining outerportions of the claddings retain a higher doping level, for example,2−5×10¹⁷ cm³.

The thickness of the lower-doped portion of the cladding can depend onthe emission wavelength, and thicker low-doped regions can be used atlonger wavelengths. In a gain medium having this feature in accordancewith the invention, the lower-doped region can be thick enough that nomore than 0.5% of the lasing mode overlaps the higher-doped region ofthe cladding.

In addition, in some embodiments, the total thickness of the bottomcladding can also be increased, typically to >4 μm, in order to avoidany loss to the high-index substrate modes. The thickness of the topoptical cladding layer is less critical, although it can be great enoughto avoid excessive loss due to overlap of the optical mode with thecontact metallization.

In some embodiments, the ratio of the field intensity in the lasing modeat the top of the bottom cladding to that at the bottom of the bottomcladding can be less than 10⁻⁵.

In addition, the higher-doped and lower-doped portions of the opticalcladding layers can be separated by one or more boundary layers in whichthe doping level is gradually graded from lower to higher doping. Thethickness of this layer can be about 100 nm.

Thus, an interband cascade gain medium can include any one or more ofFeatures (1)-(9) described above to achieve the various advantageousaspects associated therewith. As described below, these advantages havebeen confirmed by the results of recent testing of interband cascadelasers that employ gain media incorporating one or more of the featurescomprising the present invention.

In one exemplary embodiment in accordance with the present invention, a5-stage interband cascade laser (ICL) emitting at λ=3.75 μm recentlyoperated in a continuous-wave (cw) mode up to a maximum temperature ofT_(max)=319 K. This ICL incorporated all 8 features of the presentinvention, although the SCLs were much thinner than the thicknessesdescribed above with respect to Feature (8), namely 0.2 μm rather thanthe 0.5-1 μm described therein. The ICL was patterned into a 9-μm-wide,3 mm-long ridge with a high-reflectivity coating on one of the facets.Next, it was covered with a gold electroplating designed to improve theheat dissipation and mounted epitaxial side up. The device produced over10 mW of cw power at room temperature. The cw threshold voltage at 300 Kwas 2.49 V, which implies a parasitic voltage drop of 0.81 V and a“voltage efficiency” of 67%. The cw light-current-voltagecharacteristics for this device for several operating temperatures nearambient are shown in FIG. 3. ICLs having two other ridge widths, 5 μmand 11 μm, also achieved room-temperature cw operation.

At cryogenic operating temperatures, lasing threshold current densitiesas low as 2 A/cm² were observed. A maximum cw wall-plug efficiency of32.4% was estimated for a broad-area 0.5 mm-long ICL without anyelectroplating. A maximum cw output power of 1.8 W was observed for abroad-area 4 mm-long ICL.

Pulsed testing of other ICLs optimized according to some or all of thefeatures of this invention disclosure has shown the potential forachieving comparable cw operating characteristics over a wide wavelengthrange of at least 2.9-4.2 μm. The lifetimes and Auger coefficients wereextracted by correlating the measured pulsed threshold current densitiesand slope efficiencies of broad-area ICLs with calculations of opticalgain vs. carrier density in the active gain region. The internal losswas estimated using the experimental slope efficiencies. The extractedAuger coefficient was found to be relatively independent of the exactsplit between the internal efficiency and internal loss.

The inventors found that all of the data points with 2.9 μm<λ<4.2 μm hadAuger coefficients in the 4−6×10⁻²⁸ cm⁶/s range. The results are shownin FIG. 4, along with the data for optically pumped type-II lasers andtype-I semiconductor materials. The corresponding threshold currentdensities at 300 K also do not display much variation, and are as low as415 A/cm² for an ICL emitting at λ=3.2 μm. The lowest internal loss of≈6 cm⁻¹ (assuming an internal efficiency of 64% from a cavity-lengthstudy) is found for λ=3.6-4.2 μm. The loss increases by ≈50% as λapproaches 3 μm. An Auger coefficient of 7.1×10⁻²⁸ cm⁶/s and a higherinternal loss of 31 cm⁻¹ are observed for an ICL emitting at λ=5.0 μm.

Although particular embodiments, aspects, and features have beendescribed and illustrated, it should be noted that the inventiondescribed herein is not limited to only those embodiments, aspects, andfeatures.

For example, relatively small (<40%) fractions of In can be added to theGaSb hole wells and AlSb barriers, and small amounts of Sb (<30%) can beadded to the InAs electron wells without altering the basic principlesof the invention.

More than two hole wells can be used in the hole injector, and thethickness of each hole well can be increased beyond the value given inthe described embodiments. Similarly, the composition and thickness ofthe barriers in the hole injector can be changed somewhat withoutcritically affecting the device operation. The thickness and the Infraction of the GaInSb active hole well can be increased further, ifgrowth conditions permit, or decreased slightly.

The doping in the claddings, SCLs, and transition regions can be changedsomewhat while retaining a relatively low internal loss, and the amountof the overlap of the higher-doped region with the mode can be increasedor decreased slightly.

The thicknesses and compositions of the layers in the transition regionscan be changed, as long as the position of the conduction band minimumin the transition is still intermediate between those of the twofunctional regions of the device.

One, two, or more SCL regions can be employed. In almost any layer whereGaSb,

GaInSb, or AlSb is specified above, related binary, ternary, quaternary,or quinternary alloys based on the Ga(InAlAs)Sb system may usually besubstituted, as long as the primary function of a “barrier”, “holequantum well”, etc., continues to be satisfied. The same is true ofInGaAlAsSb rather than InAs or Al(In)Sb.

The structure may also include a bottom contact layer, if both top andbottom contacts are to be made from the epitaxial side of the device. Orelectrical contacts may be made without any distinct contact layer(s)being present.

Finally, as noted above, while NRL testing has confirmed that all 8 ofthe features described above are generally beneficial to the laseroperation, it may often be possible to maintain attractive ICLperformance by using any one or more of the 8 features, either alone orin combination with any other of the 8 features.

It should be readily appreciated that modifications may be made bypersons skilled in the art, and the present application contemplates anyand all modifications within the spirit and scope of the underlyinginvention described and claimed herein. All such combinations andembodiments are within the scope of the present disclosure.

1. An interband cascade gain medium, comprising: an active gain region,the active gain region having a first end and a second end opposite thefirst end and having a cladding layer at each of the first and secondends, the active gain region comprising a plurality of cascading stages,each of the cascading stages including: an active gain quantum wellregion; a hole injector region adjacent the active gain quantum wellregion; and an electron injector region adjacent the hole injectorregion; each of the active gain quantum well region, the hole injectorregion, and the electron injector region comprising a plurality ofelectron barriers and at least one of an electron quantum well and ahole quantum well, the active gain region having an active gain quantumwell region of a first stage at the first end thereof and an electroninjector region of a last stage at the second end thereof; and furthercomprising at least one high-refractive-index separate confinement layercomprising one of GaSb, GaInSb, GaAlSb, GaAsSb, GaAlAsSb, GaInAsSb,GaAlInSb, and GaAlInAsSb and having a total thickness of at least about0.8 μm disposed at least one of the first and second ends of the gainmedium between the active gain region and a corresponding claddinglayer.
 2. The interband cascade gain medium according to claim 1,wherein a total thickness of at least one electron injector region of atleast one stage of the active gain region is less than about 250 Å. 3.The interband cascade gain medium according to claim 2, wherein the holeinjector region comprises two hole quantum wells, each of the two holequantum wells comprising one of GaSb, GaInSb, GaAlSb, GaAsSb, GaAlAsSb,GaInAsSb, GaAlInSb, and GaAlInAsSb, a total thickness of the two holequantum wells being greater than about 100 Å.
 4. The interband cascadegain medium according to claim 2, wherein the active gain quantum wellregion is separated from the adjacent hole injector region by anelectron barrier having a thickness sufficient to lower a square of awavefunction overlap between a zone-center active electron quantum welland injector hole states to not more than 5%.
 5. The interband cascadegain medium according to claim 1, wherein the hole injector regioncomprises two hole quantum wells, each of the two hole quantum wellscomprising one of GaSb, GaInSb, GaAlSb, GaAsSb, GaAlAsSb, GaInAsSb,GaAlInSb, and GaAlInAsSb, a total thickness of the two hole quantumwells being greater than about 100 Å; and wherein at least one holeinjector region of at least one stage of the active gain region isseparated from an adjacent electron injector region by an electronbarrier comprising one of AlSb and AlGaInAsSb and having a thickness atleast about 20 Å.
 6. The interband cascade gain medium according toclaim 1, wherein the active gain quantum well region is separated fromthe adjacent hole injector region by an electron barrier having athickness sufficient to lower a square of a wavefunction overlap betweena zone-center active electron quantum well and injector hole states tonot more than 5%; and wherein at least one hole injector region of atleast one stage of the active gain region is separated from an adjacentelectron injector region by an electron barrier comprising one of AlSband AlGaInAsSb and having a thickness at least about 20 Å.
 7. Aninterband cascade gain medium, comprising: an active gain region, theactive gain region having a first end and a second end opposite thefirst end and comprising a plurality of cascading stages, each of thecascading stages including an active gain quantum well region, a holeinjector region adjacent the active gain quantum well region, and anelectron injector region adjacent the hole injector region, each of theactive gain quantum well region, the hole injector region, and theelectron injector region comprising a plurality of electron barriers andat least one of an electron quantum well and a hole quantum well, theactive gain region having an active gain quantum well region of a firststage at the first end thereof and an electron injector region of a laststage at the second end thereof; wherein at least one hole injectorregion of at least one stage of the active gain region is separated froman adjacent electron injector region by an electron barrier comprisingone of AlSb and AlGaInAsSb and having a thickness at least about 20 Å;and wherein a total thickness of at least one electron injector regionof at least one stage of the active gain region is less than about 250Å; and wherein the hole injector region comprises two hole quantumwells, each of the two hole quantum wells comprising one of GaSb,GaInSb, GaAlSb, GaAsSb, GaAlAsSb, GaInAsSb, GaAlInSb, and GaAlInAsSb, atotal thickness of the two hole quantum wells being greater than about100 {acute over (Å)}.
 8. An interband cascade gain medium, comprising:an active gain region, the active gain region having a first end and asecond end opposite the first end and comprising a plurality ofcascading stages, each of the cascading stages including an active gainquantum well region, a hole injector region adjacent the active gainquantum well region, and an electron injector region adjacent the holeinjector region, each of the active gain quantum well region, the holeinjector region, and the electron injector region comprising a pluralityof electron barriers and at least one of an electron quantum well and ahole quantum well, the active gain region having an active gain quantumwell region of a first stage at the first end thereof and an electroninjector region of a last stage at the second end thereof; wherein atleast one hole injector region of at least one stage of the active gainregion is separated from an adjacent electron injector region by anelectron barrier comprising one of AlSb and AlGaInAsSb and having athickness at least about 20 Å; and wherein a total thickness of at leastone electron injector region of at least one stage of the active gainregion is less than about 250 Å; and wherein the active gain quantumwell region is separated from the adjacent hole injector region by anelectron barrier having a thickness sufficient to lower a square of awavefunction overlap between a zone-center active electron quantum welland injector hole states to not more than 5%.